A major capital expense of a rectification plant for the separation of air into components based on their relative volatility is the cost of the column casing and the space required for the column. This is particularly the case where two or more columns are required to conduct the separation. Such multi-column systems are often used in cryogenic rectification, such as in the cryogenic rectification of air, where columns may be stacked vertically or located side by side. It would be highly desirable to have a system which will enable rectification to be carried out with reduced column cost and with reduced space requirements for the columns.
Divided-wall columns have been proposed in the literature as a means to better utilize a given column diameter, and thereby reduce the capital cost associated with construction of a plant to facilitate the separation process. Divided-wall columns essentially contain multiple distillation sections at the same elevation within a single column shell. An early example of the use of a divided-wall column is disclosed in U.S. Pat. Nos. 5,946,942 and 6,023,945 (Wong, et al.) discloses an application of divided-wall principles to air separation. These prior art systems disclose an apparatus wherein the lower pressure column contains an inner annular wall. The region contained between the inner annular wall and the outer shell of the lower pressure column constitutes a section for the production of argon product.
Drawbacks of the prior art divided-wall column systems include various structural and performance compromises made relating to design challenges, including: (i) maldistribution of vapor within the different sections of the divided wall column; (ii) maldistribution of the down-flowing liquids due to the large wall surface areas, particularly where structured packing is employed as the mass-transfer elements; (iii) lower performance of the divided wall columns and column internals due to transient thermal expansion/contraction differences between the inner shell and outer shell; and (iv) inadequacy of a pressure boundary between the interior core column section and annulus column region of the annular divided wall columns.
Accordingly it is an object of this invention to provide an annular divided wall column system for rectification of air which address the above-identified design challenges and overcomes the difficulties and disadvantages of the prior art annular divided wall columns to provide better and more advantageous performance.